1. Field of the Invention
This application relates to hydraulic vane pumps and more particularly to reversible pumps which can be used in either the motor or pump mode.
2. Background Art
The key to the design of a highly efficient hydraulic pump/motor is to optimize the following characteristics:
a) low internal leakage by minimizing the size and number of moving parts, controlling clearances by using precision manufacturing processes, and adding auxiliary seals as required;
b) maintaining minimum operating axial clearances by utilizing pressure compensation to balance clamping and separating forces;
c) low mechanical friction resulting from balancing forces where possible, maximizing rotor and vane rigidity, utilizing rolling element bearings where possible to replace sliding elements, and minimizing sliding velocities which is accomplished by minimizing the size of the physical components;
d) low fluid flow restriction; and
e) maximizing fluid power capacity for a given mass and package sizexe2x80x94use of light weight materials where possible.
In addition, many applications require full adjustment of fluid displacement, and even over-center adjustment to facilitate the transition from pumping to motoring and vice versa or to accommodate reverse rotation without redirection of external fluid lines and with or without reversal in direction of fluid flow.
In exploring the field of available fluid power devices used primarily in high pressure applications for industrial, agricultural, and construction industries, it has not been possible to find a design which meets all of the above criteria which would be suitable for an automotive application for regenerative braking and acceleration. Because the energy storage device for the regenerative system is an accumulator with a piston acting on compressed nitrogen, it is not possible to control the operating pressure in and out of the accumulator. Therefore, to modulate braking and acceleration forces, a variable displacement pump/motor is used to accomplish a driver controllable torque level from the near constant pressure regenerative storage device.
Currently, the highest efficiency commercially available variable displacement hydraulic devices are bent shaft and wobble plate axial piston units which are relatively massive and expensive to manufacture. To meet the compact packaging criteria above, it is felt that a new design light weight high efficiency hydraulic vane pump/motor is desirable having high pressure capability.
This invention is a reversible variable displacement hydraulic dual-pressure compensated roller tipped vane pump and motor featuring a variable depth vane slot with tailored outer ring contour having over-center stroke adjustment. The unique features of the design are listed below:
Balanced pressure compensation in both pump and motor mode by utilizing a pressurized end plate with two compensation areas to apply clamping forces proportional to the two operating pressures (inlet and outlet), thus compensating for the internal separating forces.
Further fine tuning of the pressure balance by adding communication passages between the variable pressure portion of the swept volume and small compensation pistons which adjust the clamping force to the variable separating force based on the angular position of the rotor. With small numbers of vanes, five or seven for example, the separating force varies, and therefore the required clamping force must adjust, by more than 15% based on the position of the vane as it moves through the pressure transition areas.
Ports in each of the end plates communicate with the swept volume of the vanes (outer ports) as they pass through segments 1 and 3. As a result of the vanes sweeping through the increasing and decreasing radial spaces, fluid flow is generated, which flow if resisted causes a pressure increase. This is the primary flow generating action of the pump, with the resulting flow passing through the outer ports.
There are additional ports (inner ports) which communicate with the inner extremity of the vane slots in all 4 segments. By having individual ports at the inner extremity of the vane slots in segments 1 and 3, it is possible to use the radial motion of the vanes in the slots as piston pumps to add significantly to the primary pumping action. Therefore, in both segments 1 and 3, the inner and outer ports are connected to each other. However, in segments 2 and 4 where there are no outer ports, there is maximum pressure unbalance on the vane and there can be minor amounts of radial motion depending on the ring configuration and displacement setting. Here, inner ports are required, fed by shuttle valves which supply the higher of the two pressures from segments 1 and 3, to insure good vane contact with the outer ring, thus minimizing fluid leakage across the side loaded vane.
The vane slots are stepped to extend into the integral shaft and rotor in the shape of a xe2x80x9cTxe2x80x9d, with a deep excursion in the center and shallow portions on each side for each of the radial or angled slots between the two end plates. The xe2x80x98Txe2x80x99 shape of the vane and slots allows for maximum radial dimension of the vane in the deep excursion area to support the vane in all positions of extension. The shallow portions of the rotor slot add structure to the rotor body, stiffening the rotor and thus decreasing its deflection under the cantilever loading of the vanes as they are side loaded by the pressure differential.
A corresponding inner contour of the vane allows it to clear the stiffened rotor, thus allowing a long vane stroke for a given rotor diameter. This results in increased fluid displacement for a given package size while distributing the load transfer from the integral shaft to the rotor to the vane to the fluid with minimum distortion and stress concentration. In hydraulic devices, both friction losses and fluid leakage increase as the third power of the linear dimension, so that decreases in package size (diameter) have very significant improvements in operating efficiencies.
A hydrodynamically supported roller bearing is located at the tip of each vane parallel to the axis of rotation to minimize friction by building a film of oil to keep the roller supported in its pocket.
Vanes can be spring loaded radially outward to enhance low speed operation when centrifugal force is minimum.
Radial lightening holes can reduce vane mass to decrease speed bias holes are filled with plastic to reduce losses caused by fluid compression in the clearance volume.
Slots can be slanted to minimize radial sliding friction in the preferred direction of rotation as the vanes move radially in and out under load.
Full over-center stroke adjustment of the pivoting outer ring is controlled by a stepping motor and a lead screw or equivalent to accommodate transition from pumping mode to motoring mode without requiring a four way valve to reverse the external pressure connections.
Because the unit has the capability to operate both as a pump and as a motor, and is reversible in direction of rotation, and is variable in displacement, the outer ring which controls vane travel can be adjusted in its position relative to the rotor and housing. The further off center the ring is adjusted from the center of the rotor, the greater the fluid displacement for one revolution of the rotor. By adjusting the ring from one extreme position past center to the opposite extreme position, feed ports are essentially reversed in their function, so that, for a given direction of rotation and a given fluid connection, the unit operation switches from a fluid pump to a fluid motor or vice versa. As an example, port A, connected to a lower pressure source, communicates with an increasing volume inlet segment during pumping mode and a decreasing volume discharge segment during motoring mode.
Likewise, port B, connected to a higher pressure, communicates with a decreasing volume outlet segment during pumping mode and an increasing volume inlet segment during motoring mode. When direction of rotation reverses, for pumping mode, port A becomes the high pressure discharge port, and port B becomes the lower pressure inlet port.
The outer ring has four segments which approximate four quadrants. The contour of the outer ring controls the vane motion, so it is important that its configuration be tailored when possible to the expected job and duty cycle under which it will operate. The contour can be optimized for one operating mode, and still accommodate the other modes at slightly reduced efficiency as will be described subsequently.
If the unit operates primarily as a pump at its maximum displacement with a given direction of rotation, then the outer ring can be configured to favor these conditions. In the configuration for this example, segment 1 is increasing radius (vane moving outward), segment 2 is constant radius (vane fully extended but not moving radially), segment 3 is decreasing radius (vane moving inward), and segment 4 is decreasing radius for the first half of the segment and increasing radius for the latter half of the segment. A second option allows for segment 4 to be constant radius in this preferred operating mode where it is operating as a pump at maximum displacement. If it is desired to have the direction of flow reverse as the direction of rotation reverses, then nothing changes in terms of outer ring position. As direction of flow reverses, inlet (suction) and outlet (pressure) ports also interchange with each other.
In the opposite extreme position of the outer ring required for motoring mode forward rotation, segment 1 is decreasing radius (vane moving inward), segment 2 is decreasing radius for the first half of the segment and increasing radius for the latter half of the segment, segment 3 is increasing radius (vane moving outward) and segment 4 is constant radius (vane fully extended but not moving radially).
With the second option above where segment 4 was constant radius in the pumping mode, segment 4 is now decreasing radius for the first half of the segment and increasing radius for the latter half of the segment. The choice of the two options will be based on the customer application such as whether or not the motoring is important, thus optimizing the contour to fit the highest priority duty cycle usage.
As the rotor turns, all of the pressure unbalance on any given vane is during the portion of the time when it passes through segments 2 and 4. In the full displacement position, pumping, the vane is fully extended in segment 2 and fully retracted in segment 4. Therefore, friction losses are minimized in both maximum displacement settings because there is little or no radial vane movement when the vane is highly side loaded. For volume settings less than the maximum, there is a small amount of inward movement during the first half of the segment 2 in pumping m ode and segment 4 in motoring mode, and outward movement during the second half of the same segments. With this in mind, port openings in segments 1 and 3 can be positioned to allow controlled pressure increase or decrease as the contained fluid moves from low pressure to high pressure or vice versa as it moves across segments 2 and 4.
Rotor slot distortions can be predicted from the vane force and pressure distributions. To avoid binding a vane, additional clearance may be required, primarily at the outer diameter. This means that it may be desirable to taper the slot, in the order of 0.10 (6xe2x80x2) to accommodate rotor and vane deflections without binding the vane in the slot.
Looking at the pressure gradient around th e circumference of the outer ring for the preferred direction n of rotation, there are two segments of approximately 720 duration (5 vane design) in both pumping mode and motoring mode where virtually all of the pressure differentials occur between fluid inlet and outlet. By concentrating on these two critical areas to control end clearance between the rotor and vane with the end plates, the end plates can be relieved in the noncritical areas. The leakage can be controlled without increasing the friction as much as would be experienced by tightly clamping the entire circumference.
These xe2x80x9cpinchxe2x80x9d segment plateaus in either the parent material or low friction laminate an be accomplished by a number of methods such as spraying, masking and dipping, or plating, or by removing metal in the non critical segments by mechanical or chemical machining.
Minimum restriction of fluid flow is accomplished by utilizing large and uniform passages from the external fluid fittings through the hub, seal ring junctions, the connecting outer ports in the end plate, and to the vane-swept cavities and the inner ports. If there is a preferred direction of rotation in the pumping mode, then the size of the inlet (suction) passages can be favored over the size of the outlet (pressure) passages.
Using the pump/motor as a regenerative braking device attached to the transmission of an automotive vehicle, it is anticipated that most or all of the use will be in the forward direction of rotation. The dual area pressure compensation allows for a reversal of pressures between pumping and motoring. However, in the application of the device as a pump for a four wheel drive assist, there could be considerable need for operation in the reverse direction of rotation with subsequent reversal of flow. (This synchronizes direction of rotation between front and rear wheels.) The dual area pressure compensation on the pressure plate allows for this reversal of pressure and suction ports. Friction in the reverse rotation mode will be equal to forward rotation if the rotor slots are radial. If the slots are canted, there is a slight increase in sliding friction in reverse rotation, and sliding velocities of the vane to rotor increase in both directions of rotation. This increase in friction with canted slots can result in minor decreases in efficiency in reverse rotation operation, but offset by an increase in efficiency in the total forward rotating regenerative operation, in both pumping and motoring modes, and in the forward rotation mode of the four-wheel drive assist.